# std::ios_base::sync_with_stdio

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< cpp‎ | io‎ | ios base

static bool sync_with_stdio( bool sync = true ); |  |

Sets whether the standard C++ streams are synchronized to the standard C streams after each input/output operation.

The standard C++ streams are the following: std::cin, std::cout, std::cerr, std::clog, std::wcin, std::wcout, std::wcerr and std::wclog.

The standard C streams are the following: stdin, stdout and stderr.

For a standard stream str, synchronized with the C stream f, the following pairs of functions have identical effect:

1) std::fputc(f, c) and str.rdbuf()->sputc(c).
2) std::fgetc(f) and str.rdbuf()->sbumpc().
3) std::ungetc(c, f) and str.rdbuf()->sputbackc(c).

In practice, this means that the synchronized C++ streams are unbuffered, and each I/O operation on a C++ stream is immediately applied to the corresponding C stream's buffer. This makes it possible to freely mix C++ and C I/O.

In addition, synchronized C++ streams are guaranteed to be thread-safe (individual characters output from multiple threads may interleave, but no data races occur).

If the synchronization is turned off, the C++ standard streams are allowed to buffer their I/O independently, which may be considerably faster in some cases.

By default, all eight standard C++ streams are synchronized with their respective C streams.

If this function is called after I/O has occurred on the standard stream, the behavior is implementation-defined: implementations range from no effect to destroying the read buffer.

### Parameters

sync | - | the new synchronization setting

### Return value

Synchronization state before the call to the function.

### Example

Run this code

    #include <cstdio>
    #include <iostream>

    int main()
    {
        std::ios::sync_with_stdio(false);
        std::cout << "a\n";
        std::printf("b\n");
        std::cout << "c\n";
    }

Possible output:

    b
    a
    c

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 49 | C++98 | it was unspecified (1) which state is actually returned and (2) what does 'synchronized' between standard C and C++ streams mean | both specified

### See also

coutwcout | writes to the standard C output stream stdout (global object)
cerrwcerr | writes to the standard C error stream stderr, unbuffered (global object)
clogwclog | writes to the standard C error stream stderr (global object)

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